Problem: 4 notebooks cost $6.44. Which equation would help determine the cost of 13 notebooks?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We can write the fact that 4 notebooks cost $6.44 as a proportion: $\dfrac{4}{\$6.44}$ Let $x$ represent the unknown cost of 13 notebooks. Since 13 notebooks cost $x$ , we have the following proportion: $\dfrac{13}{x}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{4}{\$6.44} = \dfrac{13}{x}$